This invention relates generally to knives of the type having a razor blade as the cutting portion and more particularly to a utility knife having an extendable and retractable blade which snaps back into the handle when the user releases his finger from an external thumb piece or knob. There have been many utility knives provided in the prior art. Such knives usually include a casing having a slidable razor blade therein with an external thumb piece connected to an internal carriage whereon the blade is mounted. Using the thumb piece, the user can slide the blade in and out of the casing through a slot at one end of the casing for the purpose of opening cardboard cartons and the like.
However, the knives are as sharp as a razor and such prior art knives are found to be dangerous and may accidentally cut the person using the knife because once the blade projects to the extent desired, the blade stays in its exposed position until it is deliberately and manually retracted.
An improvement in such construction is disclosed in U.S. Pat. No. 4,139,939, issued Feb. 20, 1979, for a utility knife which provides spring-loading on the carriage, such that when the blade has been extended from the casing by operation of the thumb piece, and the user releases the thumb piece, the blade snaps back automatically into the casing where it is completely concealed.
Nevertheless, this improved utility knife still has a major disadvantage in that a knife which is conveniently used by a right-handed person, is extremely inconvenient, in fact awkward, to use by a left-handed person. This occurs because the thumb piece lies on the broad, generally flattened side surface of the knife casing. This side surface is generally parallel to the plane of the razor-sharp blade and the forward end slot. Such flattened side surfaces of the casing make for a firm and comfortable grip.
Heretofore, manufacturers of utility knives of the type disclosed in the patent mentioned above, have provided the market with two models, a right-handed and a left-handed model, these models being mirror images of each other. The need for two models creates tooling problems for the manufacturer and inventory problems for the manufacturer, wholesalers and retailers.
What is needed is a universal utility knife with a razor-blade type cutting blade which retracts automatically into the casing and which can be used with equal facility by right-handed and left-handed persons.